Offshore hydrocarbon drilling and producing operations are typically conducted from either a bottom-founded offshore platform or a floating platform. As used herein, a "bottom-founded offshore platform" is any offshore platform which extends from the floor of the body of water upwardly to a deck located above the surface of the body of water and in which at least a portion of the weight of the platform is supported by the subsea foundation, and a "floating platform" is any ship, vessel, or other structure (e.g., a tension-leg platform) in which the weight of the platform is supported by buoyancy. The equipment and procedures used in installing a well on a bottom-founded offshore platform vary significantly from those used in installing a well on a floating platform. The principal reason for this variation is that some means must be provided for supporting the in-water portion of the well string which may be 2500 feet or more in length. On a bottom-founded offshore platform, guide sleeves are typically spaced along the length of the in-water portion. These guide sleeves provide lateral support for the in-water portion and are spaced at intervals which will prevent it from buckling under its own weight and the weight of its contents. Accordingly, the in-water portion of the well strings for a bottom-founded offshore platform are typically axially (i.e., vertically) self-supporting. On a floating platform, it is not practical to provide lateral support for the in-water portion of the well strings. Therefore, some method for maintaining the risers in tension must be provided in order to prevent them from buckling. Suitable methods include attaching buoyancy means to the upper end of the riser or using a pneumatic or hydraulic riser tensioner, well known in the art, located on the floating platform to maintain an upward load on the riser.
Well systems for bottom-founded offshore platforms traditionally include an outer casing string, known as a "conductor" (also known as a "drive pipe"), extending from the deck of the platform downwardly through the water and into the earth below the seafloor. All subsequent drilling operations for the well are performed through this conductor. Typically, conductors are driven down to a depth where the surrounding soils are cohesive enough that they will not slough into the open hole during subsequent drilling operations. The formation below the conductor should also be strong enough so that it will not fracture during subsequent drilling operations. As is well known in the art, the hydrostatic pressure exerted by a column of drilling fluid extending upwardly to the deck of the offshore platform can be high enough to fracture shallow formations which can result in lost returns and potentially in loss of the well. This problem can be especially severe in deep waters where the hydrostatic pressure of the drilling fluid becomes quite large.
Following installation of the conductors, drilling for the next casing string commences. During this drilling operation, drilling fluid and drill cuttings are typically returned at the deck of the offshore platform. This requires that significant volumes of drilling fluid be circulated down the drill string and then up the annulus between the drill string and the conductor in order to keep the drill cuttings in suspension as they are transported up the conductor. Accordingly, the speed of the drilling operation is limited by the cuttings transport capability of the drilling fluid circulation system. Again, this problem is especially severe in deep waters.
Well systems for bottom-founded offshore platforms typically include a gas diverter under the rig floor. This is because it is not uncommon for shallow pockets of natural gas (or other gases) to be encountered during the initial stages of drilling. If this should occur, the gas can flow up the conductor to the rig floor where it can be a potential safety hazard.
Conductors for bottom-founded offshore platforms are traditionally installed by a floating construction vessel after installation of the platform jacket. The conductors may be as much as 36 inches or more in diameter, and a typical platform has a plurality (up to 48 or more) of wells. As water depth increases, the material and installation costs of the conductors for a platform can become prohibitive. Also, the offshore platform itself must be strong enough to resist the hydrodynamic loads resulting from waves and ocean currents impacting the conductors. Due to the size and number of the conductors, the cost of the additional steel required to resist these loads can be significant.
Installation of a bottom-founded offshore platform can be an expensive and risky operation. Traditionally, the platform jacket is constructed at an onshore fabrication yard, transported to the installation site on a barge and offloaded, and then uprighted by flooding a portion of the legs and gradually lowered to the seafloor. Once the platform jacket is in place on the seafloor, a plurality of foundation piles are assembled, driven into the earth, and attached to the platform jacket. Assembly of the foundation piles requires that successive sections of large diameter pipe be connected end-to-end by welding or mechanical connectors as the pile is gradually lowered toward the seafloor. The pile driving operation is typically performed from one or more expensive offshore construction vessels and can take several days to accomplish. During this period it is possible that a severe storm (e.g., a hurricane) could arise. If a storm arises before the platform has been made "storm-safe", the entire platform jacket could be lost. Alternatively, the foundation piles may be assembled, as described above, and then installed by drilling and grouting, as is well known in the art. Further, it is possible that the foundation piles could be pre-installed (see U.S. Pat. No. 4,669,918 to Riles) thereby reducing the time needed to achieve a "storm-safe" condition.
Following installation of the platform jacket and the foundation piles, the same construction vessels are typically used to install the platform conductors. This is accomplished in a similar manner to installation of the foundation piles. The conductors are gradually assembled by welding or mechanically joining sections of pipe end-to-end, lowering the assembled pipe sections through the guide sleeves, and then driving them into the earth. Alternatively, in certain types of soils it may be necessary to drill a borehole into the earth and then grout (cement) the conductor in place.
It will be apparent from the foregoing that installation of a bottom-founded offshore platform and its associated wells is a complex, time-consuming, and expensive process, especially in deeper waters. Accordingly, a need exists for a method of installing a bottom-founded offshore platform and its associated wells which overcomes the problems described above.